sustainability

Article

Visualization and Analysis of Mapping KnowledgeDomain of Urban Vitality Research

Jian-gang Shi, Wei Miao * and Hongyun Si

School of Economics and Management, Tongji University, Shanghai 200092, China; sjg126com@126.com (J.-g.S.);sihongyun@tongji.edu.cn (H.S.)* Correspondence: miaowei@tongji.edu.cn

Received: 14 December 2018; Accepted: 11 February 2019; Published: 14 February 2019�����������������

Abstract: Mapping knowledge domain (MKD) is an important research technology in bibliometrics.It provides a visual perspective for researchers and helps researchers to clearly understand thegeneral situations of specific research fields. Presently, MKD has been widely used in fields such asecology and transportation safety. In this paper, we conduct a systematically bibliometric analysisof the development trend on urban vitality research by using VOSviewer software, and this workis based on the related articles published in the Science Citation Index Expanded (SCIE) and SocialSciences Citation Index (SSCI) between 1998 and 2018. In the implementation, we firstly introduce theannual numbers of articles, source journals, highly cited papers, and production countries relatingto urban vitality research. Then, we adopt a keywords co-occurrence analysis to explore the topicdistribution, research frontiers, and research trends of urban vitality research intuitively. Overall, theMapped Knowledge Domain method has proved to be an effective way to assess research trends inspecific disciplines and to contribute to researchers and governments with respect to managementand decision-making on issues of science.

Keywords: urban vitality; bibliometric analysis; VOSviewer; visualization; mappingknowledge domain

1. Introduction

The history of urban development has exceeded a thousand years. Especially after the eighteenthcentury, city development has entered a rapid stage. Urban spaces are the carriers of human activities,and the development of cities is closely related to the future destiny of mankind. But what kindof city can be considered as a successful one? This is a question that urban researchers have beenexploring for a century. Some literature illustrates that a successful city should make its residentsfeel safe, comfortable and full of vitality [1,2]; have a rich street life, and have a reasonable urbanorder and urban form which can satisfy the needs of various human activities [3]. The city shouldhave five characteristics: Vitality, suitability, sense, accessibility, and controllability [4]. Among thesefeatures, vitality means that the city provides residents with a safe place to live which can satisfy allthe necessary activities for them [5].

Urban vitality is the most basic element for achieving the quality of life in the city. Therefore,an increasing number of scholars are interested in the study of urban vitality. According to Jacobthe “liveliness and variety attract more liveliness; deadness and monotony repel life” [6,7]. Lynchsupplemented this idea, believing that urban vitality consists of three main parts: Urban morphology,urban function, and urban society [8]. In his paper, Maas described urban vitality as three aspects: thepersistence of people in a public space, their activities, and opportunities, and the place where theseactivities take place. Maas believed that these are the three basic aspects of community-scale urbanvitality [9]. At the same time, March et al. pointed out that the measurement of urban vitality should

Sustainability 2019, 11, 988; doi:10.3390/su11040988 www.mdpi.com/journal/sustainability

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take into account the range of experience needed for a healthy lifestyle, including privacy, rest and,meditation [10].

However, how to measure and evaluate the intensity of urban vitality has always been a difficultproblem in this field. Existing studies have tried to solve this problem indirectly from differentperspectives. For example, some scholars evaluated the vitality of cities from the accessibility of retailstores, transportation, and jobs in cities [11,12]; there are scholars who set their sights on the naturalenvironment [13,14] and socio-economic environment [15,16]; other research perspectives includeurban landscape aesthetics, urban infrastructure, and social interaction [17]. These findings really letreaders have a further understanding of urban vitality.

Therefore, in order to better understand the research themes and trends in a certain scientific field,it is necessary to conduct a systematic and comprehensive analysis of global scientific production [18].Before this study, many scholars in other disciplines have used this strategy to find their researchpriorities [19]. In this sense, the bibliometric analysis will play an important role in scientificdecision-making [20]. Bibliometrics is a statistical technology which includes scientific publications,citations, patent documents, and reports [21,22]. Because this technology can integrate and analyzeall publicly published documents in a particular research field or discipline [23–25], it can reducethe scientific boundaries between countries or regions [26], and also enable researchers to clearlyunderstand the cooperative relationship between literature authors. Therefore, bibliometrics hasreceived special attention from the scientific community [27]. In the current body of research, there isa lack of work to apply this technology to analyze the current situation and development trends ofurban vitality research.

The remainder of this paper is arranged as follows. Section 2 describes the materials andmethods used. In Section 3, we introduce the results of the analysis, including the number ofpublications per year, source journals, influential productive countries, and keywords co-occurrencenetworks. In Section 4, the results are discussed. Section 5 concludes the paper and draws somemeaningful conclusions.

2. Research Methods and Processes

2.1. Data Collection

The Science Citation Index Expanded (SCIE) and Social Sciences Citation Index (SSCI) citationindex databases in the Web of Science (WoS) core collection were retrieved as the source databasefor this study, which is a common source of data for scientific research [28,29] and bibliometricresearch [21,22,30]. The search expressions included the application of “Urban Vitality” to the “Topic”field, which retrieves the title, abstract, and keywords (including the keywords PLUS) of publicationsin WoS to find the required literature [31]. The time span was “1998 to 2018”, and the publication typeselected “Article” and “Review” because they represent most of the literature with complete researchresults [31,32]. After the search, a total of 436 eligible documents were collected, and the search resultswere saved as “Other File Formats” with “Full Record and Cited References”, and the data was savedon 25 November 2018.

2.2. Scientometric Analysis

Urban vitality is a multidisciplinary research field involving the integration of economics,sociology, and engineering. There are few studies on the current research status of urban vitalitydone through bibliometric analysis. Bibliometrics is a statistical technique that includes scientificpublications, citations, patent documents, and reports. This technology has been widely used in fieldssuch as ecology and transportation safety. It should be noted that bibliometric analysis is a quantitativeanalysis process, but the results are qualitative [33]. In recent years, many bibliometric tools have beendeveloped by researchers such as Bib Excel, Ucinet, SCIMAT, VOSviewer, and CiteSpace. Some ofthese software (Bib Excel, Ucinet) are specialized in data processing, but they need to import the results

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into other visualization software (Pajek, Gephi) to display network maps, while SCIMAT, VOSviewer,and CiteSpace can not only deal with data but also display the results.

The main research method of this paper is based on Scientometrics, and the analysis software usedin this paper includes VOSviewer (version 1.6.9; www.vosviewer.com) and MS Excel. VOSviewer isdeveloped by van Eck and Waltman of Leiden University in The Netherlands. It can analyze complexnetworks with its own clustering analysis function according to the connection strength betweenprojects [34]. At the same time, the software uses Visualization of Similarities (VOS) technology andhas strong advantages in mapping knowledge domain display, especially in the aspect of clustering.In general, VOSviewer is a powerful visualization software of document knowledge units, especiallysuitable for analyzing large-scale data sets and constructing complex networks. Therefore, VOSviewerhas been widely used in bibliometrics. In the latest version 1.6.9 of VOS viewer, file formats such as.net, .mat, and .txt can be used to construct keywords co-occurrence networks, reference networks, andcoupling networks.

3. Results

3.1. Evolution of Scientific Activity: Prior Analysis

Urban vitality is not a new research field. In traditional research, urban vitality is regarded asa part of urban planning research. It is considered as including the energy and power to promoteurban development [35] and the basic elements for realizing the quality of urban life [36]. In recentyears, there has been a growing number of studies on urban vitality. The research topics involve urbanenvironment [37,38], geographic science [39,40], human travel behavior [41,42], and other fields, thusforming a wide and diversified academic research system. The change in the number of academicpapers for a particular research topic is an important indicator for analyzing the trend of the researchtopic. Simultaneously, this indicator also reflects the change in the degree of knowledge of the subject.Statistical analysis of the number of papers published in WOS every year can clarify the currentresearch status and future development trend of this area. The trends in the number of research paperson urban vitality from 1998 and 2018 are summarized in Figure 1.

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publications, citations, patent documents, and reports. This technology has been widely used in fields such as ecology and transportation safety. It should be noted that bibliometric analysis is a quantitative analysis process, but the results are qualitative [33]. In recent years, many bibliometric tools have been developed by researchers such as Bib Excel, Ucinet, SCIMAT, VOSviewer, and CiteSpace. Some of these software (Bib Excel, Ucinet) are specialized in data processing, but they need to import the results into other visualization software (Pajek, Gephi) to display network maps, while SCIMAT, VOSviewer, and CiteSpace can not only deal with data but also display the results.

The main research method of this paper is based on Scientometrics, and the analysis software used in this paper includes VOSviewer (version 1.6.9; www. vosviewer.com) and MS Excel. VOSviewer is developed by van Eck and Waltman of Leiden University in the Netherlands. It can analyze complex networks with its own clustering analysis function according to the connection strength between projects [34]. At the same time, the software uses Visualization of Similarities (VOS) technology and has strong advantages in mapping knowledge domain display, especially in the aspect of clustering. In general, VOSviewer is a powerful visualization software of document knowledge units, especially suitable for analyzing large-scale data sets and constructing complex networks. Therefore, VOSviewer has been widely used in bibliometrics. In the latest version 1.6.9 of VOS viewer, file formats such as .net, .mat, and .txt can be used to construct keywords co-occurrence networks, reference networks, and coupling networks.

3. Results

3.1. Evolution of Scientific Activity: Prior Analysis

Urban vitality is not a new research field. In traditional research, urban vitality is regarded as a part of urban planning research. It is considered as including the energy and power to promote urban development [35] and the basic elements for realizing the quality of urban life [36]. In recent years, there has been a growing number of studies on urban vitality. The research topics involve urban environment [37,38], geographic science [39,40], human travel behavior [41,42], and other fields, thus forming a wide and diversified academic research system. The change in the number of academic papers for a particular research topic is an important indicator for analyzing the trend of the research topic. Simultaneously, this indicator also reflects the change in the degree of knowledge of the subject. Statistical analysis of the number of papers published in WOS every year can clarify the current research status and future development trend of this area. The trends in the number of research papers on urban vitality from 1998 and 2018 are summarized in Figure 1.

Figure 1. Quantitative distribution of published articles in urban vitality studies, 1998–2018.

The trend shown in Figure 1 demonstrates how research about urban vitality has attracted theattention of many scholars around the world. In terms of the number of publications, before 2006,

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there were less than 10 papers per year on urban vitality. However, by 2018, there were more than50 papers on urban vitality, and the numbers are still being updated. This trend shows that the fieldof urban vitality will be an emerging research topic and will be involved in more fields. The reasonfor the growth of research papers about urban vitality may be that people are paying more attentionto improving quality of life, and they are looking forward to living in a livable and vibrant city. Therelevant research about urban vitality is to explore how cities can make life better.

3.2. Main Journals and Papers with the Most Impact

The 437 documents retrieved were published in 293 different publications. However, as this is anemerging field, the total amount of literature on urban vitality research in 1998–2018 is small, resultingin the scattered distribution of these documents in nearly 300 journals. It should be pointed out, byunderstanding that the fields of papers received by these journals include urban studies. As listed inTable 1, among the top 10 most active journals, Cities is the top 1, a well-known urban research journalin America. But the total number of research papers about urban vitality published in Cities so far isless than 20. Even so, it is believed that more papers on urban vitality will be published in this journalin the future.

Table 1. Top 10 journals ranked by total publications, 1998–2018.

Rank Journal Quantity

1 Cities 142 Urban Forestry & Urban Greening 103 Urban Design International 94 Sustainability 75 PloS One 66 Transportation Research Record 67 Habitat International 58 Journal of Urban History 59 City Community 410 Environmental Pollution 4

The second-ranked magazine is Urban Forestry & Urban Greening, the areas covered by which arePlant Science, Environmental Science & Ecology, Forestry, and Urban Studies. Up to now, 10 researchpapers have been published in the magazine. The rest of journals in Table 1 are Urban DesignInternational (9 papers), Sustainability (7 papers), PloS One (6 papers), Transportation Research Record(6 papers), and Habitat International (5 papers). The distribution pattern of the papers published inthese journals is decreasing, which is expected in bibliometrics (Bradford method), because only a fewjournals are dedicated to specific topics, although many other journals may publish scientific paperson the subject.

By analyzing the 10 most cited papers about urban vitality published on WoS, it is clear fromTable 2 that the highly cited papers in this field are not closely related to the most active journals inthis field. Only two papers in this table are published in the publications listed in Table 1. The mostfrequently cited article was “The Change Metabolism of Cities”, published in the Journal of IndustrialEcology, which was not listed in the top 10 active journals. From the perspective of urban metabolism,this paper analyses the spatial relationship between urban vitality and surrounding regions and theglobal resource network and explores the manner of urban sustainable development [43]. This thesisstudy involved the city’s water cycle and energy circulation, resulting in a higher number of referencesthan other papers in Table 2.

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Table 2. Top 10 most cited papers, 1998–2018.

Rank Title Journal Year Quantity

1 The changing metabolism of cities Journal of Industrial Ecology 2007 445

2 Urban development and the politics of acreative class: evidence from a study of artists Environment and Planning A 2006 376

3Effectiveness of counselling patients on

physical activity in general practice: clusterrandomized controlled trial

British Medical Journal 2003 303

4

Incorporating thermal regimes intoenvironmental flows assessments: modifying

dam operations to restore freshwaterecosystem integrity

Freshwater Biology 2010 282

5 The trajectories of European cities, 1960–2005 Cities 2007 219

6 Amenities drive urban growth Journal of Urban Affairs 2002 188

7 The influence of urban green environmentson stress relief measures: A field experiment

Journal of EnvironmentalPsychology 2014 140

8 Role of environmental estrogens in thedeterioration of male factor fertility Fertility and Sterility 2002 139

9’Grandparents are the next best thing’:

Informal childcare for working parents inurban Britain

Journal of Social Policy 2002 139

10Outdoor thermal comfort and outdooractivities: A review of research in the

past decadeCities 2012 121

Other papers shown in Table 2 are also analyzed, for instance, because some scholars believethat cities are the places where economic and social vitality takes place, and the decline of the urbanpopulation will seriously affect the economic and social vitality of cities [44,45]. At the same time,the diversity of urban residents’ occupations has made an important and positive contribution to thevitality of the city [46]. Some scholars also pay attention to the important role of cultural activities inurban economic vitality [47]. Urban social vitality will have an important impact on the ecologicalenvironment [48], but it also has a close relationship with the health of residents [49–52].

3.3. Quantitative Analysis of Productive Countries

According to the search results, all the research papers on urban vitality come from 59 countries(or territories) in the world. Table 3 lists the top 10 productive countries (or territories) with a totalnumber of 338 academic papers which accounts for 77.5% of the total publications. Most articles arepublished by developed countries in North America and Europe, including the United States, England,and The Netherlands. Among them, the United States is the most active country in urban vitalityresearch, with 128 papers published, accounting for 29% of the total. Secondly, China has become anemerging and competitive country in scientific research in the world, publishing 48 articles, accountingfor 11% of the total; England ranks third, contributing 40 articles, accounting for 10% of the total;Australia and The Netherlands are the fourth and fifth, respectively, with 23 papers. The averagenumber of papers published in countries between the 6th and 7th places is 3% of the total. The cities ofthese developed countries are either highly modernized or in stages of rapid expansion, so the studyof urban vitality is very important for them.

The retrieved publication data are shown as labels and thermographic patches in Figure 2. Thecolor of the patches depends on the number of papers published by the countries (or regions) whichare located at the center of the patches. The larger the number, the warmer (redder) the color of thepatches; conversely, the smaller the number, the colder (bluer) the color of the patches. As shown inthe figure, in this density view, countries with a large number of published papers are at the center ofthe plaque.

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Table 3. Top 10 productive countries in urban vitality studies, 1998–2018.

Rank Countries/Territories Region Quantity

1 USA North America 1282 Peoples R China Asia 483 England Europe 404 Australia Oceania 235 The Netherlands Europe 236 Italy Europe 197 Germany Europe 158 Spain Europe 159 Iran Central Asia 14

10 Portugal Europe 13

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published by developed countries in North America and Europe, including the United States, England, and the Netherlands. Among them, the United States is the most active country in urban vitality research, with 128 papers published, accounting for 29% of the total. Secondly, China has become an emerging and competitive country in scientific research in the world, publishing 48 articles, accounting for 11% of the total; England ranks third, contributing 40 articles, accounting for 10% of the total; Australia and the Netherlands are the fourth and fifth, respectively, with 23 papers. The average number of papers published in countries between the 6th and 7th places is 3% of the total. The cities of these developed countries are either highly modernized or in stages of rapid expansion, so the study of urban vitality is very important for them.

Table 3. Top 10 productive countries in urban vitality studies, 1998–2018.

Rank Countries/Territories Region Quantity 1 USA North America 128 2 Peoples R China Asia 48 3 England Europe 40 4 Australia Oceania 23 5 Netherlands Europe 23 6 Italy Europe 19 7 Germany Europe 15 8 Spain Europe 15 9 Iran Central Asia 14

10 Portugal Europe 13 The retrieved publication data are shown as labels and thermographic patches in Figure 2. The

color of the patches depends on the number of papers published by the countries (or regions) which are located at the center of the patches. The larger the number, the warmer (redder) the color of the patches; conversely, the smaller the number, the colder (bluer) the color of the patches. As shown in the figure, in this density view, countries with a large number of published papers are at the center of the plaque.

Figure 2. The density of main research countries in urban vitality studies, 1998–2018.

Figure 2 clearly shows the three labels of the United States, China, and England in a prominent position on the map. According to the statistics in Table 2, the United States contributed a third of the total publications of the literature, and in fact, the color of the plaque in the United States is also the

Figure 2. The density of main research countries in urban vitality studies, 1998–2018.

Figure 2 clearly shows the three labels of the United States, China, and England in a prominentposition on the map. According to the statistics in Table 2, the United States contributed a third of thetotal publications of the literature, and in fact, the color of the plaque in the United States is also thedeepest. The reason for the leading number of urban vitality research papers in the United States maybe that the United States has two world-class megacities in Los Angeles and New York. In addition,there are other big cities also located in the United States. Therefore, it seems that American scholarsare more urgent about the study of urban vitality. Another reason may be that after the September11 attacks, the US government has higher requirements for the safety and resilience of the city. Thisshows that the United States is indeed the center of urban vitality research in the world. In recent years,Chinese cities are in the stage of rapid expansion, and increasing new urban districts have been set up.This has led Chinese scholars to pay more attention to the study of urban vitality. In Figure 2, the patchcolor of China is second only to that of the United States. London is one of the world-class cities. Thequestion of how to achieve the sustainable development of the city and make London more livable hasbecome a problem explored by scholars. Driven by these problems, England has made many scientificand practical achievements in the field of urban vitality research. England’s label is clear in Figure 2,and other countries, such as Australia, The Netherlands, Italy, Germany can be seen in Figure 2, whichshows that the research results of these countries in the field of urban vitality are also important.

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3.4. Keywords Co-occurrence Analysis: Main Topic Distribution and Research Fronts of Urban Vitality

The keywords are an important part of a research paper, which carries important informationabout the paper [53,54]. Systematic analysis of keywords in specific research fields can clearlyunderstand the development trends and research differences in the field [55]. Keywords co-occurrenceanalysis is often used to analyze the strength of links between different keywords in a large number ofdocuments. By analyzing the co-occurrence relationships of the keywords, we can clearly understandthe internal composition relationship and structure in a certain academic domain and reveal theresearch frontiers of the discipline. Consequently, keywords co-occurrence analysis has become acommon research method in Scientometrics.

A total of 2334 keywords appeared in all the collected literature on urban vitality, which wouldbe recognized by VOSviewer and displayed in the co-occurrence network ordinarily. But in order tounderstand the relationship among the documents, a selection condition was set in VOSviewer, whichmeans that keywords with a frequency of more than 5 appearances can be recognized by the software.The result of the screening was that a total of 94 qualified keywords were successfully identified by thesoftware. Then, we got a keywords co-occurrence network with 851 links on urban vitality research,as shown in Figure 3. In Figure 3, each node represents a keyword, the size of the node indicatesthe number of occurrences of the keyword, and the link connecting of the two nodes represents therelationship between the two keywords.

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Figure 3. The keywords co-occurrence network of urban vitality studies.

Cluster 1 (red): All the keywords of cluster 1 are listed in Table 4, in which “growth” appears the most, with a total of 13 occurrences, followed by “quality”, while “air-pollution”, “responses”, and “biomonitoring” are ranked 3rd–5th, respectively.

On the hand, “growth” in this cluster means the patterns of growth of a city. For example, He used association rule analysis to explore the relationship between different urban growth patterns on urban vitality, and the results indicated that different forms of urban growth have different effects on urban vitality [36]. On the other hand, “growth” refers to the growth of urban trees [56,57]. These keywords also include “drought”, “tree vitality”, “vegetation”, and “forest”. From this research perspective, these keywords seem to have no direct connection with urban vitality. However, in fact, the vegetation and green infrastructure in cities are considered as important components of the cityscape [58,59], and they are of great significance to people living in cities. At the same time, plants can create suitable places for human activities and can alleviate air pollution in the city, which is conducive to residents' outdoor activities, leisure, and social interactions [60], effectively enhancing the vitality of the city [37,61].

Table 4. The keywords information for cluster 1.

Number Keywords Occurrences Total Link Strength 1 growth 13 32 2 quality 11 40 3 air-pollution 10 34 4 responses 9 24 5 biomonitoring 8 23 6 diversity 8 19 7 drought 8 19

Figure 3. The keywords co-occurrence network of urban vitality studies.

It can be clearly seen that the research theme of urban vitality has roughly formed six clusters inFigure 3, and there is a significant correlation between the keywords in each cluster. According to thecurrent situation of urban vitality research, the six clusters were analyzed as follows.

• Cluster 1 (red): All the keywords of cluster 1 are listed in Table 4, in which “growth” appears themost, with a total of 13 occurrences, followed by “quality”, while “air-pollution”, “responses”,and “biomonitoring” are ranked 3rd–5th, respectively.

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Table 4. The keywords information for cluster 1.

Number Keywords Occurrences Total Link Strength

1 growth 13 322 quality 11 403 air-pollution 10 344 responses 9 245 biomonitoring 8 236 diversity 8 197 drought 8 198 dynamics 5 119 heavy-metals 8 2110 climate-change 7 2511 tree vitality 7 812 vegetation 7 2113 chlorophyll fluorescence 6 1714 forest 6 1815 management 6 1116 air pollution 5 1717 ecosystem services 5 1418 location 5 2119 plants 5 1220 trees 5 9

On the hand, “growth” in this cluster means the patterns of growth of a city. For example, He usedassociation rule analysis to explore the relationship between different urban growth patterns on urbanvitality, and the results indicated that different forms of urban growth have different effects on urbanvitality [36]. On the other hand, “growth” refers to the growth of urban trees [56,57]. These keywordsalso include “drought”, “tree vitality”, “vegetation”, and “forest”. From this research perspective, thesekeywords seem to have no direct connection with urban vitality. However, in fact, the vegetation andgreen infrastructure in cities are considered as important components of the cityscape [58,59], and theyare of great significance to people living in cities. At the same time, plants can create suitable places forhuman activities and can alleviate air pollution in the city, which is conducive to residents’ outdooractivities, leisure, and social interactions [60], effectively enhancing the vitality of the city [37,61].

• Cluster 2 (green): The most frequently used keyword for this cluster is “city”, where the citymeans a place where human activities can be satisfied. The same keywords include “cities”,“urban”, “space”, “public space”, “neighborhoods”, and “urban space” in Table 5. It is worthnoting that keywords in a paper are often distributed in two or more clusters, which result in aninteresting phenomenon in keywords co-occurrence networks; that is, there is a certain degree ofoverlap between different clusters.

Maintaining community vitality has become an important strategy for urban development [62],while outdoor public spaces are the most dynamic places in the city. Public space in cities, whichare available to varying extents in all urban areas, can be used for outdoor activities, leisure andsocial interactions [60], which helps to improve the quality of life in urban areas and thus effectivelyenhances the vitality of cities [37,61]. On the other hand, the vitality and function of spaces largelydepend on the comfortable conditions of people in these places [60]. Specifically, urban public spaceincludes urban green space [37], historical blocks [63], walking streets [64,65], and so on; however, thecity is a complex entity, and the efficiency of an urban public space is limited by many factors suchas climate [62,66,67], vegetation [58], and historical buildings [62]. The issue of how to successfullytransform the potential of public space use into high-quality life is an arduous task. Zhang used spatialsyntax to describe the changes of the street structure in Shichahai Road, Beijing, and the results showedthat Beijing is increasingly dependent on automobiles, which has reduced the vitality of Beijing to a

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certain extent [68]. These research results promote the trend of sustainable development of cities andbecome a scientific reference for policymakers.

Table 5. The keywords information for cluster 2.

Number Keywords Occurrences Total Link Strength

1 city 40 1022 vitality 26 513 cities 24 794 urban 14 405 space 13 316 public space 11 187 policy 9 248 culture 8 89 sustainable development 7 1210 geography 6 1511 neighborhoods 6 1312 politics 6 1313 technology 6 1614 urban space 6 615 gentrification 5 1816 infrastructure 5 1617 space syntax 5 518 urban development 5 8

• Cluster 3 (blue): The keywords (Table 6) of this cluster almost all appear in the relevant research onurban form and neighborhood vitality. In recent decades, neighborhood vitality has also receiveda growing amount of attention [69–71], and urban form is considered to be an effective way topromote neighborhood vitality [72,73]. Mixed land use, as the most typical feature of urban form,is the principal factor to be solved in the study of neighborhood vitality [42]; other characteristicsinclude block density and accessibility [9,74,75]. These characteristics reflect differences in urbanneighborhood characteristics in different urban forms, and these differences may have directimpacts on urban vitality [76]. Mixed land use can effectively promote neighborhood vitality,which is widely accepted in urban planning [77,78]. Similarly, traffic accessibility and block densitycan also be used to promote and maintain urban vitality [79]. At the same time, studies haveshown that improving community vitality is an effective approach to reducing crime rates [80],alleviating gender discrimination [81], and enhancing community resilience [82].

• Cluster 4 (yellow): In Table 7, the keywords for the “built environment” appear most frequently.The “built environment” refers to places provided for human activities between buildings,including streets, green space, and other outdoor public space. In fact, all human physical activitytakes place in the built environment. Conversely, it is precisely because of human activities thatit brings vitality to the city and promotes economic growth and social development of the city.In academia, many documents describe the complex relationship between built environment,physical activity, and public health [83–87]. The urban building environment is characterizedby high density and mixed land use, and the good street network between buildings providesspace for walking and other physical activities [88,89]. The urban building environment containsa variety of human activity information [90], which usually exists in mobile location [91,92]and social media data with geotags [93,94]. By collecting and extracting these characteristicsinformation (such as time, age, gender group, and region), the distribution of people in urbanspaces can be revealed, and the relationship between human diversification behavior and urbanvitality can be established [95]. These studies showed that good street networks and comfortablepublic environments are closely related to urban vitality [96], which can help guide policymakersand urban planners to conduct their actions and enhance street vitality through renovating thebuilding environment.

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• Cluster 5 (purple): The most frequent keyword of this cluster is “China”, followed by “quality oflife” “population” in the Table 8, which may be because many scholars are very interested in theurban development of China as a developing country, and are especially interested in the specificpopulation demographics in Chinese cities, including women, the elderly, and rural migrants.Improving the quality of life and living environments of these people is the most significantbenefit of improving urban vitality. Studies had demonstrated that by improving the vitality ofurban space through relevant measures, the number of women in this area will increase, and theirquality of life will also be improved [97]. For urban migrants and the elderly, the same resultsapply. Therefore, it is also important to analyze the comfortable conditions of urban public spacesfor the vitality of urban spaces.

• Cluster 6 (orange): The number of keywords in this cluster is the least of all; there are only sixkeywords (Table 9). As can be seen in Figure 3, the uniqueness of this cluster does not seem to besignificant but is embedded in other clusters. The reason for this phenomenon may be becausethe keywords in cluster 6 and the keywords in other clusters have a strong correlation in meaning.For example, the keyword “health” with the highest number of occurrences in cluster 6 refersto the health status of residents. It is well known that “green space” and “urban environment”can improve the quality of life of residents [98], alleviate the working pressure of urban residents,and then improving the health level of residents [99–101]. Obviously, these keywords are closelyrelated to “built environment” (cluster 4), “quality of life” (cluster 5), “public space” (cluster 2),“vitality” (cluster 2), “satisfaction” (cluster 5), “neighborhood” (cluster 4), and they often appearin the same paper.

Table 6. The keywords information for cluster 3.

Number Keywords Occurrences Total Link Strength

1 environment 17 502 community 16 283 sustainability 16 344 place 9 205 urban regeneration 8 146 governance 7 117 model 7 158 travel 7 309 accessibility 6 1610 areas 6 1911 climate change 6 1812 migration 6 1313 transportation 6 2014 urban form 6 2815 urban planning 6 1016 crime 5 1217 gender 5 1918 resilience 5 8

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Table 7. The keywords information for cluster 4.

Number Keywords Occurrences Total Link Strength

1 Built Environment 14 682 Physical-activity 14 513 Urban Vitality 13 414 Impact 12 255 Neighborhood 11 546 United-States 11 307 Design 10 398 Behavior 9 309 Land-use 8 4010 Urban Design 8 3411 Walkability 6 3112 Walking 6 2713 Walking Activity 6 3414 Density 5 1815 Determinants 5 1916 Older-adults 5 17

Table 8. The keywords information for cluster 5.

Number Keywords Occurrences Total Link Strength

1 China 19 602 Quality of Life 13 463 Quality-of-Life 13 354 Population 8 305 Sf-36 8 196 Adults 7 307 Satisfaction 7 188 Mortality 6 219 Risk 6 1210 Validity 6 2411 Women 6 9

12 Health-related Quality ofLife 5 15

13 Outcomes 5 1514 People 5 1815 Reliability 5 2016 Survey Sf-36 5 21

Table 9. The keywords information of cluster 6.

Number Keywords Occurrences Total Link Strength

1 Health 18 462 Stress 13 363 Benefits 7 284 Exposure 6 145 Green space 5 266 Urban Environment 5 15

It is not clear why VOSviewer classified these keywords into the same category and formed anew cluster instead of assigning them to other similar clusters. This reason needs to be explored infuture work.

4. Further Discussion

Generally speaking, urban vitality has attracted more and more attention in academic circles. Asdescribed in this paper, urban vitality has formed a wide and diverse literature knowledge system.

Sustainability 2019, 11, 988 12 of 17

In terms of the number of publications, papers on urban vitality are showing a rapid growth trend,which may be caused by the acceptance of the concept of sustainable development across the world.In another sense, the increase in the number of papers reflects the requirements of urban residentsnecessary to improve the quality of life.

In terms of research topics, unlike traditional research on urban vitality, some scholars haveconducted micro-observations on urban vitality by using new geographic data and have achievedmany surprising research results in recent years. For example, social media registration data canreflect the population distribution and mobility in urban spaces, which has been widely used toanalyze patterns of human mobility [102,103] and urban structures [104–106]. In the existing studies,the contribution of urban morphology to neighborhood vitality has not been fully or systematicallymeasured, especially in Chinese cities. Because streets and public spaces are objectively physicalplaces and do not have life characteristics, it follows that only users living in public spaces can reflectthe vitality of streets and public spaces. Some scholars used GPS-facilitated activity diary data toanalyze the impact of the urban form on neighborhood vitality. From the perspective of residents’non-working activities around their neighbors, GPS-facilitated activity diary data can reflect the realinteraction between residents and neighbors [42]. In addition, these data can also be used to analyzethe socio-economic characteristics of residents to show how different types of residents can promotethe vitality of neighborhoods under the influence of the urban form. In summary, although theresearch topics in the field of urban vitality are very diverse, scholars prefer the research topics ofurban morphology, geographic data, human travel behavior, and so on. These topics have become hottopics in the study of urban vitality.

5. Conclusions

In conclusion, this paper evaluates urban vitality research from 1998–2018 through bibliometricanalysis in order to quantitatively understand the literature trends related to the theme. The MKDanalysis method based on Scientometrics has advantages of visualization, quantitative analysis, andknowledge discovery. This paper uses VOSviewer to carry out a multi-angle and complementaryanalysis of urban vitality research. The main conclusions are as follows:

• The increase in the number of articles indicates that the study of urban vitality in the globalacademic community is developing rapidly. According to the distribution of articles in differentcountries, the United States, China, and England rank in the top three, which indicates thatthese countries are the development centers and active areas of urban vitality research. In termsof source journals, Cities, Urban Forestry & Urban Greening, and Urban Design International areauthoritative journals in the field of urban vitality research. They are important platforms forpublishing and exchanging research results.

• Through the co-occurrence of keywords in the clustering network map and the analysis of urbanvitality research activities, it is found that the frontier topics of urban vitality research includesix representative areas. There are some overlaps among the six clusters, which shows that thereare some similarities among different research topics. This result confirms that urban vitality is across-disciplinary research field.

In this study, we focus on the research topics and frontiers of urban vitality. We propose asystematic analysis based on MKD to determine the knowledge base, topic distribution, and researchtrends. In future research, the development of branches or themes of urban vitality research willbe further studied. In the meanwhile, other bibliometric analysis tools (such as Citespace) willbe considered in combination with the results of this paper. In addition, the weight of keywords,authors, and source journals will be the emphasis of the next study to make the results of this papermore meaningful.

Sustainability 2019, 11, 988 13 of 17

Author Contributions: The research was designed and performed by J.-g.S. and W.M. The paper was written byJ.-g.S. and W.M., and finally checked and revised by H.S. All authors read and approved the final manuscript.

Funding: This research received no external funding.

Acknowledgments: The authors are very grateful for the valuable comments and suggestions of the anonymousreviewers, which significantly improved the article.

Conflicts of Interest: The authors declare no conflict of interest.

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